Reduction of microfoam in a spray-applied waterborne composition

Abstract: A method for reducing the amount of microfoam in a spray-applied waterborne polymeric composition is provided, wherein a gas, such as, for example, carbon dioxide, having a solubility factor greater than one is used in spraying the waterborne composition. Articles so coated are also provided. And, a method for increasing the density of a spray-dried waterborne polymeric composition is provided, wherein a gas, such as, for example, carbon dioxide, having a solubility factor greater than one is used in spray-drying the waterborne composition.

Description: This invention concerns a method for reducing the formation of microfoam in spray-applied, waterborne polymeric compositions. The invention further concerns the use of a gas having a solubility factor of greater than one as the carrier gas in the spray application of a waterborne polymeric composition. Waterborne polymeric compositions, such as clear or pigmented coatings, are frequently applied to substrates by spraying techniques. Some of the common spraying techniques used are, for example, air spray, airless spray, air-assisted airless spray, disc and bell spray, and high volume low pressure spray. In air spray, compressed air is used to atomize the waterborne polymeric composition and to assist in conveying the droplets to the substrate. In airless, disc, and bell spray techniques the waterborne polymeric composition is atomized by mechanical means and the droplets are suspended in air on formation. Air-assisted airless spray is a hybrid of the two and involves the use of air as a carrier gas to convey spray droplets which are formed mechanically. High volume, low pressure spray is another commonly recognized variant of air spray. In each case, air is involved either in the atomization of the polymeric composition, or in the conveyance of the droplets to the target substrate or both. As a result of the spraying technique, coatings formed by spray-applying waterborne polymeric compositions onto the surface of a substrate frequently contain microfoam which remains in the dried film and detracts from the appearance of the film, particularly from the appearance of clear, or unpigmented, film. It is the object of the present invention to reduce the amount of microfoam in spray-applied, waterborne polymeric compositions. In the prior art, US-A-4,842,900 discloses a method and apparatus for coating a liquid coating composition onto a moving web and in which air entrained on the web is replaced by a gas having a higher solubility in the liquid coating composition than air, such as, for example, carbon dioxide. US-A-3,661,605 discloses an aqueous aerosol composition comprising a homogenized dispersion of lecithin in water and pressurised carbon dioxide or nitrous oxide as the propellant. The composition is used to produce release coatings of lecithin on a surface of a substrate. US-A-4,869,878 discloses a device for the creation of an oxygen-free working atmosphere in applications where molten metal or ceramic is spray applied to a workpiece and which makes use of, for example, combustion gas either as the propellant gas for the spray, or simply as a buffer. US-A-4,923,720 discloses a process in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce the consistency viscous coatings compositions to allow for their application as liquid sprays. US-5,164,177 discloses a non-foaming aqueous aerosol hair spray composition consisting essentially of a fixative resin, a water-soluble electrolyte, water, 0-30% alcohol, and a propellant gas. GB-A-2,020,308 discloses a mixture of carbon dioxide, propane and butane as a propellant for aerosol spray cans, particularly lacquer spray cans. In JP-A-60-85929 a method is disclosed for the spray-application of a temporary, water soluble protective coating onto the surface of a FRP (fibre reinforced plastic) to protect that surface from contact with atmospheric oxygen during the cure of the FRP. Following the cure the temporary protective coating is removed by washing. The coating composition used comprises a water-soluble resin, e.g. vinyl acetate resin, and an oxygen-blocking agent such as silicone oil. The propellant gas used in spray process is either a fluorocarbon or carbon dioxide. In JP-A-62-252702 an insecticidal aerosol composition is described containing a water-soluble polymer, e.g. vinyl acetate, as a component and utilising carbon dioxide inter alia as the propellant. Such compositions have nothing to do with the present invention. None of that art deals with the associated problems of microfoam formation in a spray-applied waterborne composition. In accordance with the present invention, we have now discovered that the problem is solved by using as the carrier gas a gas having a solubility factor (as defined below) greater than 1, preferably greater than 35. Preferably the gas used is nitrous oxide or carbon dioxide, most preferably the latter. Accordingly, in accordance with the first aspect of the present invention there is provided a method for reducing the formation of microfoam in spray-applied polymeric compositions, which method comprises spraying on to the surface of a substrate a waterborne polymeric composition comprising an emulsion polymerised polymer and wherein the carrier gas used in the spraying operation is a gas having a solubility factor of greater than one. In the second aspect of the present invention, there is provided the use of a gas having a solubility factor of greater than one as the carrier gas in the spray-application of a waterborne polymeric composition on to the surface of a substrate for reducing the amount of microfoam in the spray-applied, waterborne polymeric composition. Particular embodiments of the invention are subject of the dependent claims, respectively. As used herein the "solubility factor" of a gas is defined as: solubility factor of a gas = solubility of the gas in water solubility of nitrogen in water where solubility is measured at 250°C at 1 atmosphere (1.013 bar) gas pressure. Some solubility factors are presented below: Gas Solubility in water Solubility Factor Nitrogen 0.65 millimoles per litre - Carbon dioxide 34 millimoles per litre 52.3 Nitrous oxide 24 millimoles per litre 36.9 A "waterborne polymeric composition" herein is defined as a composition containing either a soluble polymer dissolved in or an insoluble polymer dispersed in a medium which is predominantly composed of water, or mixtures thereof. The waterborne polymeric composition may contain a solution of a polymer in water or in an aqueous medium, which aqueous medium may be, for example, a mixture of water and at least one water-miscible solvent such as, for example, isopropanol, ethylene glycol butyl ether, and propylene glycol propyl ether; the waterborne polymeric composition may contain an alkali-soluble polymer dissolved in an alkaline aqueous medium; or the waterborne polymeric composition may contain a dispersion of an insoluble polymer in an aqueous medium such as, for example, an emulsion-polymerized polymer. Polymers in the waterborne polymeric compositions are generally prepared by the addition polymerization of at least one ethylenically unsaturated monomer such as, for example, esters of (meth)acrylic acid, vinyl esters, styrene, and butadiene. Polar monomers such as (meth)acrylic acid, itaconic acid, acrylonitrile, dimethylaminoethyl (meth)acrylate, and hydroxyethyl (meth)acrylate may also be incorporated in the polymer. Low levels of multi-ethylenically unsaturated monomers may be incorporated in the polymer. Chain transfer agents such as, for example, mercaptans may be used in order to moderate the molecular weight of the polymer. When the polymer is in the form of an emulsion-polymerized polymer, the particles may be from about 40 nanometers to about 4000 nanometers in diameter. The solids content of waterborne polymeric compositions may be from about 20% to about 70% by weight. The viscosity may be from about 50 centipoises to about 10,000 centipoises, as measured using a Brookfield viscometer (Model LVT using spindle #3 at 12 rpm); the viscosities appropriate for different spraying methods vary considerably. Waterborne polymeric compositions may contain, in addition to the polymer, conventional components such as, for example, emulsifiers, pigments, fillers, dispersants, coalescing agents, anti-migration aids, curing agents, thickeners, humectants, wetting agents, biocides, plasticizers, antifoaming agents, colourants, waxes, and anti-oxidants. A spray-applied waterborne polymeric composition is a composition which has been applied to a substrate such as, for example, metal, wood, and plastic, using a spraying method. Preferred is a composition which has been applied to a wood substrate such as, for example, wood, sealed wood, particle board treated with a UV-cured filler, painted wood, and previously coated wood; or to an automotive substrate such as, for example, plastic, reinforced plastic (such as RIM substrate), metal, treated metal, metal coated with an electodeposited primer, and previously painted metal; by a spraying method such as, for example, air-assisted spray, airless spray, bell or disc spraying, high velocity low pressure spray, and air-assisted electrostatic spray. In spraying methods the waterborne polymeric composition is atomized, or formed into small droplets, which are conveyed to the substrate where the droplets form into a substantially continuous structure such as, for example, a film. In such spray-applied methods the atomized droplets of the waterborne polymeric composition are formed in contact with and/or admixed with a gas - conventionally, air. The gas, under pressure, may be required to atomize the coating such as, for example, in conventional air spray applications; the gas may flow towards the substrate and provide for at least some of the conveying of the atomized composition such as, for example, in air-assisted airless spray application; or the gas may be the medium through which the atomized composition, atomized by mechanical action in the presence of the gas such as, for example, in airless spray, disc, and bell applications, with or without electrostatic assistance, moves to the substrate. Occlusions of gas, "microfoam", are typically found in the waterborne polymeric composition after its application to the substrate when using air as the gas in the process. The microfoam is undesirable; it may cause haze or opacity in clear or substantially unpigmented coatings or films; and it may lower the clarity, depth, gloss, or "distinctness of image" of clear or pigmented coatings or films. "Microfoam" herein is defined as substantially spherical gas-filled occlusions which are typically 10-20 micrometers in radius. The microfoam occlusions lack sufficient buoyancy to escape from the waterborne polymeric composition before they become substantially immobilized in the applied composition. The "amount of microfoam" as described herein is determined by counting the number of bubbles in a selected area of the applied waterborne composition, using an optical microscope under 70X magnification. In the method of this invention microfoam which has formed within the applied waterborne polymeric composition is reduced in amount by using in place of air a gas having a solubility factor greater than about one. "Using a gas having a solubility factor greater than one" as used herein means that at least one gas having a solubility factor, as defined herein, greater than about one is used in the spraying process in place of the air commonly used. The gas having a solubility factor greater than about one may be used under pressure where such is required by the spraying method selected; or such a gas may be used at atmospheric pressure in the area where the atomized droplets are formed and admixed with gas. Preferred is using a gas with a solubility factor greater than about 35. Most preferred is carbon dioxide gas. The following examples are intended to illustrate the method for reducing the amount of microfoam in a spray-applied waterborne polymeric composition. EXAMPLE 1 Reduction of microfoam when using a gas-assisted suction-feed spray gun 1000g of a styrene/acrylic emulsion polymer (Rhoplex™ CL-105) at 36% solids by weight was adjusted to pH =8.5 with 14% NH 4 OH and a solution of 46.8g ethylene glycol monobutyl ether, 54g isopropyl alcohol, 7.2g ethylene glycol 2-ethyl hexyl ether, and 276.6g water was added with stirring using a bench top stirrer to provide a sealer composition. A maple wood panel sanded with 220 grit sandpaper was sprayed with this sealer composition to a wet film thickness of 4 mils; the coating was air-dried for 45 minutes and re-sanded with 220 grit sandpaper to provide a sealed maple panel. 300g of a styrene/acrylic latex (Rhoplex™ CL-104) at 38.5% solids by weight was adjusted to pH =7.5 with 14% NH 4 OH and a solution of 17.3g ethylene glycol monobutylether, 11.5g diethylene glycol monobutylether, and 32.1g water was added with stirring using a benchtop stirrer. This topcoat composition was sprayed using a conventional suction-feed spray gun (DeVilbiss MBC) with an EX tip and a #30 air cap, with a gas pressure of 50 psi. Three top coats were applied to each sealed maple panel described herein-above with 30 minutes of dry time between each coat. One of the panels was sprayed using compressed air while the other panel was sprayed using compressed CO 2 . The panels were sprayed under the same temperature and humidity conditions (30°C/40%RH) and were dried under these conditions. This was accomplished by connecting a cylinder of CO 2 directly to the spray gun in the same manner as the compressed air. The dried panels were evaluated for distinctness of image (DOI), gloss (20°/60°/85°), visual microfoam, and amount of microfoam. Gloss readings were made with a Byk Labotron gloss unit. Distinctness of image readings were made using a Gardner DOI meter (Model # GB11-8GM) which uses a Landolt Ring film to quantify the DOI. DOI readings were made by placing the coated piece under the DOI meter and visually determining the smallest rings which could clearly be distinguished. DOI was rated from 100 (best) to 0 (worst).The panel was approximately 9mm below the light source. The amount of microfoam was determined by counting the number of bubbles in a 1 sq. mm. area of the film using an optical microscope at 70X magnification and reported as the number of bubbles in a 1 sq. mm. area. The results are presented in Table 1.1 below. Gas-assisted spray-applied clear coating evaluation Gas DOI Gloss 85°/60°/20° Microfoam Amount Air 0 24/22/4 >200 CO 2 40 81/83/50 5 Using carbon dioxide with a solubility factor greater than about one in the method of this invention caused a substantial reduction in the amount of microfoam in the dried coating and an improvement in other appearance properties of the coating. EXAMPLE 2 . Reduction of microfoam when using a gas-assisted suction-feed spray gun A sealed maple wood substrate and topcoat composition were prepared as in Example 1. The topcoat composition was sprayed using a conventional suction-feed spray gun (DeVilbiss MBC) with an EX tip and a #30 air cap, with a gas pressure of 50 psi. Three top coats were applied to each sealed maple panel with 30 minutes of dry time between each coat. The panels were sprayed under the same temperature and humidity conditions (30°C/40%RH) and were dried under these conditions. One of the panels was sprayed using compressed air while the other panel was sprayed using compressed N 2 O. This was accomplished by connecting a cylinder of N 2 O directly to the spray gun in the same manner as the compressed air. The dried panels were evaluated for distinctness of image (DOI), gloss (20°/60°/85°), and amount of microfoam using the methods of Example 1. The results are presented in Table 2.1 below. Gas-assisted spray-applied clear coatings Gas DOI Gloss 85°/60°/20° Microfoam Amount Air 0 40/30/6 >200 N 2 O 50 93/86/61 30 Using nitrous oxide with a solubility factor greater than about one in the method of this invention caused a substantial reduction of microfoam in the dried coating and an improvement in other appearance properties of the coating. EXAMPLE 3 Reduction of microfoam when using an "airless spray" spray gun A waterborne polymeric composition was prepared. The following ingredients were dispersed using a Cowles disperser at high speed. 165g Titanium dioxide (Tioxide TR-92) 31.2g Diethylene glycol monomethyl ether 6.95g Anionic pigment dispersant(Tamol 681; 35% solids) 0.8g NH 4 OH (28%) 1.5g Anionic surfactant (Triton CF-10) 0.5g Defoamer (Drew L-493) 15.7g water The following ingredients were added in the order given, with stirring. 519.9g Styrene/acrylic emulsion polymer (Experimental Emulsion E-3054PMN at 50.6% solids) 181.2g water 4g Sodium Nitrite (15% aqueous) 32.9g Texanol coalescent 7.6g Urethane rheology modifier (QR-708 ;10% aqueous) 5.2g Linoleic Acid (tech. grade) 5.2g Cobalt drier (Aquacat; 5% Co) 2.6g Manganese drier (Magnacat; 5%Mn) The pH was then adjusted to 9.5 with 14%NH 4 OH and this composition was applied to an Alodine aluminum panel by "airless spraying". A 30:1 Graco President pump and a Binks Airless 1 gun was used. The fluid pressure was 2300 psi and the tip orifice was 0.019" which gave a 10" fan. "Airless spray" relies on ejection of the coating from a small orifice at high pressure, typically at 1500-3000 psi, to atomize the composition; in this Example air or, alternatively, CO 2 gas was passed over the tip of the airless spray gun in order to affect the composition of the gas in the foam bubbles which would be present initially in the applied composition; it is believed that the foam was the result of the surrounding gas being entrained in the applied composition. The gas pressure was approximately 50psi. One coat approximately 12 wet mils in thickness was applied and the coated aluminum panel was dried at 24°C and 25%RH. The dried panels were evaluated for distinctness of image (DOI), gloss (20°/60°/85°), and amount of microfoam. Test methods used were the same as in Example 1. Results are presented in Table 3.1. Evaluation of the effect of surrounding gas composition using airless spray spray-application Gas DOI Gloss 85°/60°/20° Amount of Microfoam Air 0 77/75/36 80 CO 2 40 90/86/55 20 Using carbon dioxide with a solubility factor greater than about one in the method of this invention caused a substantial reduction of microfoam in the dried coating and an improvement in other appearance properties of the coating. EXAMPLE 4 Reduction of microfoam using a high volume low pressure (HVLP) spray gun A sealed maple wood panel was prepared as in Example 1 for use as a substrate. 300g of a styrene/acrylic latex (Rhoplex™ CL-104) at 38.5% solids by weight was adjusted to pH =7.5 with 14% NH 4 OH and a solution of 23.1g ethylene glycol monobutylether, 5.8g diethylene glycol monobutylether, and 32.1g water was added with stirring using a benchtop stirrer. This waterborne polymeric composition was sprayed using a HVLP spray gun (Binks Mach 1) with a #97 nozzle, a #95P aricap, and a gas pressure of 90 psi. Three coats were applied to each sealed maple panel with 30 minutes of dry time between each coat. The panels were sprayed under the same temperature and humidity conditions (24°C/25%RH) and were dried under these conditions. One of the panels was sprayed using compressed air while the other panel was sprayed using compressed CO 2 . This was accomplished by connecting a cylinder of CO 2 directly to the spray gun in the same manner as the compressed air. The dried panels were evaluated for distinctness of image (DOI), gloss (20°/60°/85°), and amount of microfoam. Test methods were those of Example 1. The results are presented in Table 4.1 below. Gas-assisted HVLP spray-applied clear coatings Gas DOI Gloss 85°/60°/20° Microfoam Amount Air 0 39/31/6 160 CO 2 50 84/80/54 12 Using carbon dioxide with a solubility factor greater than about one in the method of this invention caused a substantial reduction of microfoam in the dried coating and an improvement in other appearance properties of the coating. EXAMPLE 5 Reduction of microfoam when using an air-assisted airless spray gun A sealed maple wood panel was prepared as in Example 1 for use as a substrate. 300g of a styrene/acrylic latex (Rhoplex™ CL-104) at 38.5% solids by weight was adjusted to pH =7.5 with 14% NH 4 OH and a solution of 23.1g ethylene glycol monobutylether, 5.8g diethylene glycol monobutylether, and 32.1g water was added with stirring using a benchtop stirrer. This waterborne polymeric composition was sprayed using an air-assisted airless spray gun (Kremlin MR) with a #09.133 tip and a #BX3a air cap. A #807 pump was used to pressurize the liquid. The liquid pressure was 400 psi and the gas pressure was 45 psi.. Three coats were applied to each sealed maple panel with 30 minutes of dry time between each coat. The panels were sprayed under the same temperature and humidity conditions (24°C/25%RH) and were dried under these conditions. One of the panels was sprayed using compressed air while the other panel was sprayed using compressed CO 2 . This was accomplished by connecting a cylinder of CO 2 directly to the spray gun in the same manner as the compressed air. The dried panels were evaluated for distinctness of image (DOI), gloss (20°/60°/85°), and amount of microfoam. Test methods were those of Example 1. The results are presented in Table 5.1 below. Gas-assisted airless spray-applied clear coatings Gas DOI Gloss 85°/60°/20° Microfoam Amount Air 0 28/25/5 70 CO 2 10 47/42/11 30 Using carbon dioxide with a solubility factor greater than about one in the method of this invention caused a reduction of microfoam in the dried coating and an improvement in other appearance properties of the coating.

Claims: A method for reducing the formation of microfoam in a spray-applied waterborne polymeric composition, which method comprises spraying on to the surface of a substrate a waterborne polymeric composition comprising an emulsion polymerised polymer, wherein the carrier gas used in the spraying operation is a gas having a solubility factor of greater than one. A method according to claim 1, wherein the carrier gas has a solubility factor of greater than 35. A method according to claim 2 wherein the carrier gas is nitrous oxide or carbon dioxide. A method according to any one of the preceding claims, wherein the carrier gas is employed also to atomize the waterborne polymeric composition as well as to convey it to the substrate. A method according to any one of the preceding claims, wherein the substrate is wood, plastics or metal. A method according to claim 5, wherein the substrate is an automotive part. The use of a gas having a solubility factor of greater than one as the carrier gas in the spray application of a waterborne polymeric composition on to the surface of a substrate for reducing the amount of microfoam in the spray applied, waterborne polymeric composition. The use as claimed in claim 7, wherein the carrier gas has a solubility factor of greater than 35, and is preferably either nitrous oxide or carbon dioxide. The use as claimed in claim 7 or claim 8, wherein the waterborne polymeric composition comprises an emulsion polymerised polymer.

Inventors: GEBHARD MATTHEW STEWART, SIMON ETHAN SCOTT, GEBHARD, MATTHEW STEWART, SIMON, ETHAN SCOTT
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1994
Main classification: B05D1
IPC codes: B05D7, B05D1, B05B7, B05D3, C09D5, C09K3